Mosquito season in St. Clair County, MI: when it starts and ends

Mosquito months in St. Clair County

In a normal year mosquito season in St. Clair County, MI starts around April 26 and ends around October 17, about 25 weeks.

1. Jan
2. Feb
3. Mar
4. Apr
5. May
6. Jun
7. Jul
8. Aug
9. Sep
10. Oct
11. Nov
12. Dec

What to do in St. Clair County, and when

• Start tipping out standing water weekly by April 12.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 17.
• 6 West Nile cases since 2016 (last 2025): you should use repellent at dusk July to July, more so if over 60.

St. Clair County against the rest of MI

St. Clair County has the 20th longest season of 83 MI counties, at 174 days against 189 in Allegan County and 124 in Iron County. Its mean temperature is 48°F.

• Barry County: April 25 to October 16 (174 days, 2 WNV)
• Saginaw County: April 25 to October 17 (174 days, 1 WNV)
• Ottawa County: April 27 to October 19 (174 days, 6 WNV)
• Kent County: April 27 to October 17 (173 days, 46 WNV)

West Nile in St. Clair County

From 1999 to 2025, CDC ArboNET counted 11 human West Nile cases in St. Clair County (11 neuroinvasive) across 10 years. The worst was 2023 with 2; the latest was 2025.

More: how the dates are worked out · reading the West Nile counts.